Retorting oil shale with special pellets and precoking stage

ABSTRACT

Hot special heat-carrying pellets are continuously cycled to a retort zone to mix with and continuously retort fresh crushed oil shale, thereby producing carbonaceous products and spent shale. The main source of heat for retorting is derived from controlled burning of a coke-like carbon-containing deposition which is deposited on the pellets as they are cycled in the process. At least a portion of the pellets are coked to some extent by cracking or stabilizing oil products in a thermal precoking zone. Some of the pellets from the precoking zone may be cycled to the retort zone. The amount of preretort coking provides flexible regulation over the total deposition formed on the pellets during the process especially that portion of the deposition formed during the retorting of oil shale and over product quality. After retorting, the pellets passed to the retort zone are separated from substantially all of the spent shale smaller than the pellets prior to combustion of the carbon deposition on the pellets. The pellets from the precoking zone and from the retorting zone are passed or lifted to a pellet deposition burning zone where the deposition is burned. The special pellets are characterized primarily by their effective surface area, size, and quantity relative to the oil shale. The process stresses improvement in the quality of the liquid oil products and at the same time greater useful recovery of the carbonaceous matter in oil shale.

United States Patent 1191 Wunderlich et al.

[ Nov. 26, 1974 RETORTING OIL SHALE WITH SPECIAL Primary Examiner-C.Davis PELLETS AND PRECOKING STAGE [75] Inventors: Donald K. Wunderlich;James L. [57] ABSTRACT Skinner, both of Richardson, Tex. Hot specialheat-carrying pellets are continuously cycled to a retort zone to mixwith and continuously re- [73] Assxgnee' Atlanuc Rldlf'eld Company L08tort fresh crushed oil shale, thereby producing carbo- Angeles, Calif. I

naceous products and spent shale. The mam source of [22] Filed: Oct. 26,1973 heat for retorting is derived from controlled burning of acoke-like carbon-containing deposition which is [2]] Appl' 410099deposited on the pellets as they are cycled in the pro- Related US.Application Data cess. At least a portion of the pellets are coked to[63] Continuation-impart" of Ser. No. 308,136, Nov. 20, some extent byFrackmg or Stablhzing Oil products in a 1972, abando ed, which i a c iiq f thermal precoklng zone. Some of the pellets from the Ser. No.304,074, Nov. 6, 1972, abandoned, which is precoking zone may be cycledto the retort zone. The a continuation-in-part of Ser. No. 284,288, Aug.28, amount of preretort coking provides flexible regula-1972.11bafld9nedtion over the total deposition formed on the pelletsduring the process especially that portion of the depo- U-S- Cl. sitionformed during the retorting of hale and over [51] Int. Cl C10b 53/06product lit After retorting, the pellets passed to Fleld of Search t t lthe retort ane are separated from substantially all of the spent shalesmaller than the pellets prior to com- References Clled bustion of thecarbon deposition on the pellets. The UNITED STATES PATENTS pellets fromthe precoking zone and from the retorting 3.008.894 1 1 I196] Culbertson208/11 Zone are Passed Or lifted to a P deposition burning 3,013,343 119 2 Ncvens 203 1 zone where the deposition is burned. The special pel-3.020.227 2/1962 Nevens et al. 208/11 lets are characterized primarilyby their effective sur- 3,058,903 10/1962 Otis 208/11 face area, size,and quantity relative to the oil shale. 3251836 5/1966 Crawfordw 208/11The process stresses improvement in the quality of the ggz a liquid oilproducts and at the same time greater useful u-aman I 1 1803.022 4/1974AbduLRahman lllllllllllllllll H 208/ recovery of the carbonaceous matterin Oll shale.

50 Claims, 2 Drawing Figures FLUE GAS h I TION DE os BURNING n-57COMBUSTION GAS 20m:

HOT A53 67 SPECIAL PELLETS ,65 69 'x THERMALLY CRACKED PRODUCTS 1PRODUCTS PRECOKING j ZONE 73 5 PRODUCTS 71 /l 31 SHALE RETORT paooucrsA2 FEED ZONE AND PELLET SPENT SHALE SEPARATION K 47 1a ZONE 2| PELLETLIFTING PELLETS PATENTE W 3,850,739

SHEET 10F 2 FLUE GAS PELLET DEPOS'T'ON COMBUSTION GAS BURNING ZONE HOT67 SPECIAL PELLETS 69 v THERMALLY CRACKED PRQDUCTS.

33 r I PRODUCTS PRECOKING v C ZONE 49 PRODUCTS 3|/ SHALE l7 I l2 FEED SI S PRODUCTS AND PELLET SPENT SHALE 47 ggglRATlON Y 2| 4 l PELLETS a F lG. l

RETORTING OIL SHALE WITH SPECIAL PELLETS AND PRECOKING STAGE CROSSREFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of copending application Ser. No. 308,136, filedNov. 20, 1972, entitled Retorting Oil Shale with Special Pellets andPrecoking Stage, now adandoned, which was a continuation-in-part ofcopending application Ser. No. 304,074, filed Nov. 6, 1972, entitledRetorting of Oil Shale with Special Pellets and SupplementalDeposition," which is a continuation-in-part of copending applicationSer. No. 284,288, filed Aug. 28, 1972, entitled Retorting of Oil Shalewith Special Pellets, both now abandoned. All of these applications havebeen filed by the same inventors as this application and are owned by acommon assignee.

BACKGROUND OF THE INVENTION This invention relates to a process forretorting of the solid carbonaceous organic matter in crushed oil shale.In the process, special heat-carrying pellets are cycled to a thermalprecoking stage, or to a retorting stage zone, or to both stages.

As a preliminary stage in the production of petroleum oils and gases,the solid carbonaceous organic solid matter or kerogen in oil shale ispyrolyzed or retorted. In an overall commercial operation, the retortproducts are processed in additional stages, for example, solidsseparation, condensation, fractionation, coking, hydrogenation, and thelike, depending on the types of marketable products being produced. Manyprocesses have been suggested for the retorting stage of a commercialoperation. The term retorting denotes. thermal conversion of kerogen ororganic matter to oil vapors and gas thereby leaving solid particulatespent shale and includes separation of the oil vapors and gas from thespent shale. The spent shale contains residual carbonaceous organicmatter and matrix mineral matter.

Frequently, the yields of various processes are compared with FischerAssay yields. For a description of the FischerAssay refer to Method ofAssaying Oil Shale by a Modified Fischer Retort by E. Stanfield and I.C. Frost, R. I. 4477, June 1949, US. Department of Interior.

When the kerogen is retorted, a normally gaseous fraction, a normallyliquefiable vaporous fraction, and a combustible organic residue areformed. The product distribution between gas, liquid, and residue isimportant and relates to the distribution of the various boiling pointfractions in the liquid product. It is highly desirable to obtain aliquid product that is directly adaptable to prerefining and avoids orlessens the amount of residue or 975F plus fraction that must besubjected to In addition, the kerogen content of the oil shaleinherently or naturally fluctuates between rich and lean, and manyprocesses are not sufficiently flexible to control product distributionwhen the kerogen content varies.

Some advances to more flexible and efficient control over the productsof retorting and of other variables have been made by using solidheat-carrying bodies which exhibit good heat transfer properties andsupply the heat needed for retorting with a reduction in processproblems. In such processes, the heatcarrying bodies and the oil shalefeedstock are intermixed thereby retorting oil vapors, gases, andcombustible residue from the feedstock. The heat-carrying bodies areusually heated in a separate heating zone by burning combustible fuelmaterial, such as heavy resid or natural gas. But in general, thismethod of heating necessitates additional equipment and createsadditional handling problems.

Others have proposed cycling the partially spent shale and supplyingsome of the heat by burning the residual carbonaceous organic matter orsolid organic char developed in the retort zone, or cycling catalystparticles and supplying some of the heat by burning carbon deposited onthe catalyst (for example, US. Pat. No. 3,281,349). In this latterprocess, the surface area of the catalyst particles is not specified.Some types of catalyst particles frequently have high surface areaswhich result in loss of valuable liquid product and excessive gaseousproduct, excessive residue, excessive heating of the catalyst duringburning, loss of valuable heat values, higher oxygen demands, and otherdisadvantages.

In addition, a large amount of fine (e.g. minus 14 US. Standard Sievesize) spent shale is usually present during burning and reheating. Thisspent shale contains organic carbon and increases oxygen demands, causesloss of useful heat values, and adversely enlarges the size ofequipment. Fine materials also interfere with control of the burning andother stages of the process and create many other problems especiallywhen the entrained spent shale is smaller than other heatcarryingbodies. Moreover, the presence of appreciable amounts of fine spentshale severely limits the type of equipment which can be used forburning the residue. Generally, burning relatively large particles inthe presence of fine material requires the use of lift pipes. If air isused as the lift gas, this form of burning could entail a large excessof oxygen which could rapidly burn the organic matter and createdisadvantages in the process of this invention.

Copending application Ser. No. 410,200 filed Oct. 26, 1973, which is acontinuation-in-part of application Ser. No. 284,288, whichisincorporated herein, provides a process for retorting oil shale usingspecial pellets in a way which regulates the amount of combustibleorganic carbon deposition formed on the pellets during retorting of oilshale and improves the recovery of useful components and liquid productdistribution.

The process relies on the interrelation between the surface area of thepellets and other conditions and variables; however, additionalregulation and flexibility are desired primarily because it has beenfound that the retorting stage of the process requires constant controland adjustment and altering one variable affects other variables andresults and because it is desirable at times to produce higher gravityor upgraded products and/or to supply more sensible heat and depositionto the pellets. Additional process flexibility and regulation is required, for example, when the retorting stage of the process is operatedunder conditions such that the amount of deposition formed on thepellets during retorting is not sufficient to reheat the pellets. Suchconditions could arise when a vein of lean oil shale is encountered andthe design and size of the retorting equipment are such that it would beundesirable or inefficient to adjust operation to the lean shale. Thereare other similar occurrences or objectives which arise during theretorting of shale. For example, a rich vein of oil shale is also likelyto be encountered. When such occurrence takes place or when objectiveschange or fluctuate, more process flexiblity and regulation areadvantageous. Briefly, therefore, a principle object of this inventionis to provide greater flexibility and adaptability to a retortingprocess of the type disclosed in copending application Ser. No. 410,200.

SUMMARY OF THE INVENTION In a retorting process, crushed carbonaceoussolid organic matter is retorted to gas, oil vapor, and combustibleresidue with special heat-carrying pellets in a manner which emphasizesgreater utility of all three re tort products and greater usefulrecovery of the residue that is normally formed when oils are retorted,cracked or vaporized. Greater useful recovery of this residue reducesthe need for liquid fuels and residue treating processes therebyincreasing the ultimate yield of the liquid oil products in a commercialsyncrude operation. The process cycles special hot heat-carrying pelletsin a way which produces a carbon-containing deposition on the pelletsand renders the deposition useful as a fuel for heating the pellets. Inthe process some or all of this deposition is burned in a pelletdeposition burning zone to heat and reheat the pellets. Some of thisdeposition is formed on the pellets during a precoking or thermalcracking or stabilization stage in which at least a portion of thevaporous, or condensed, or condensed and fractionated retort productsare cracked or stabilized in the presence of at least a portion of thepellets to deposit a coke-like deposition on the pellets thus exposed.This thermal precoking stage is carried out after the pellets have beenreheated by combustion of the deposition on the pellets. But the thermalcracking stage is carried out before the precoke pellets, if any, arefed to the retort zone where the solid organic matter in oil shale isretorted. Since a portion of the deposition is formed on the pellets inthe precoking zone, the effective surface area of the pellets passed tothe retorting zone may be altered by passing some or all of the precokedpellets to the retorting zone. Consequently, the deposition formed onthe precoked pellets passed to the retorting zone either reduces theamount of deposition formed on the pellets in the retort or adds to theamount of deposition formed in the retort, thereby increasing the totalamount of deposition formed on the pellets. The amount of depositionformed on the pellets during the retorting stage of the process ispreferably less than 1.5 per cent by weight per pass through the retort.The pellet precoking or prethermal cracking stage thereby providesgreater regulation and greater flexibility and adaptability to theretorting process. As a side advantage, the retorting process provides away to upgrade or stabilize products by thermal cracking orstabilization and at the same time place the coke that is normallyformed during thermal cracking in a better position to be used as fuelfor reheating the pellets to retort oil shale. The pellet precoking orprethermal cracking or stabilization stage is itself flexible andadaptable since the operator has the choice of subjecting any part ofthe retort products to such prethermal cracking or stabilizationconditions, or of passing any part of the pellets through the prethermalcracking or stabilization stage. A preferred feed for the precoking zoneis oil out boiling between 100F and 700F. The quantities of pellets orproducts, or both, so treated may be varied to coact with changes inother variables especially the organic content of the raw shale and thedesired product yields. Further, since the precoking stage is carriedout while the pellets are hottest, sensible heat requirements for suchthermal cracking or stabilization are better controlled and moreflexible; moreover, the temperature of the pellets entering theretorting stage of the process is under better control in the event thatthe pellet deposition burning zone is not adequately controlled. Stillfurther advantages are available in that the burning of the depositionand the cycling of the pellets tends to reduce the effective surfacearea of the pellets. The rate of change in surface area is not constantand tends to approach an asymptotic or equilibrium value which isestablished by the nature of the pellets and the process conditions. Theflexibility provided by the precoking stage can be used to compensatefor such changes in surface area or to provide more uniform operatingconditions for the retort zone.

The special pellets are comprised of particulate or divided solid heatcarriers whose physical properties and characteristics, especiallysurface area, size, shape, temperature, and amount, coact with othervariables to control the amount of organicdeposition formed on thepellets during the process, especially during the retorting stage, andto accomplish the other objectives and advantages of the process.

In the process, mined oil shale which contains solid carbonaceousorganic matter and other mineral matter and which has been crushed andmay have been preheated is retorted in a retort zone with the specialhot heat-carrying pellets at a temperature and in an amount sufficientto provide at least 50 per cent of the sensible heat required to retortthe oil shale. Retorting oil shale produces gas and oil products, whichare recovered, and particulate spent shale. Retorting also tends todeposit a carbon-containing deposition on the special pellets.

After retorting the oil shale, at least per cent of the total spentshale and at least per cent of the spent shale smaller than the pelletsare separated from the pellets prior to burning deposition on thepellets. One way to accomplish this separation is to first screen largespent shale and agglomerates from the pellets and thereafter subject thepellets and remaining spent shale to gas elutriation with anoncombustion supporting gas. A way to enhance the degree of totalseparation is to control the sphericity factor of the pellets to atleast 0.9, or to crush the raw oil shale to a smaller than normal size,that is, to minus 6 US. Sieve Series size.

After separation of the spent shale, the pellets from the retorting zonewhich have a combustible deposition deposited thereon along with anypellets passed directly from the precoking zone are passed to a pelletdeposition burning zone where at least a portion of the deposition isburned to heat the pellets. Thereafter, at least a portion of the heatedpellets and at least a portion of the oil products are passed to aprecoking zone where the heat in the pellets causes partial thermal stabilization and cracking of oil products from the retort and causesdeposition of a combustible coke-like deposition on the pellets. Theprecoked pellets are then either passed to the retort zone or directlyback to the pellet deposition burning zone.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. 1 is a schematicflow-sheet of the process of this invention; and

FIG. 2 is a partly schematical, partly diagrammatical flow illustrationof a system for carrying out a preferred sequence of the process of FIG.1.

DETAILED DESCRIPTION OF THE INVENTION A process for retorting crushedoil shale containing carbonaceous organic matter and other mineralmatter is described in general terms having reference to FIG. 1 and inmore particular terms having reference to FIG. 2. I Raw or fresh oilshale which has been mined and pulverized, crushed or ground for themost part to a predetermined maximum size for handling in a retortsystem by any suitable particle diminution process is fed directly froma crusher or from a hopper or accumulator by way of shale inlet line 11into retort zone 13. At the same time, special heatcarrying pelletssubstantially hotter than the shale feed are fed by gravity or othermechanical means to the retort zone by way of pellet inlet pipe 15. Thepellets and shale feedstock could be fed to the retort zone by way of acommon retort zone inlet.

Crushing of the raw mined shale expedites more uniform contact and heattransfer between the shale feedstock and hot pellets. In normalpractice, the degree of crushing is simply dictated by an economicbalance be tween the cost of crushing and the advantages to be gained bycrushing when retorting the kerogen from the shaleQGenerally the shalefeedstock is crushed to about inch and no particular care is taken toproduce or restrict production of finer material. In this process,crushing has a special purpose and aids in a preburn separation step. Inone embodiment, for reasons which will be hereinafter shown and despitethe added costs and standard practice, the mined shale is crushed to asubstantially finer size wherein at least 95 per cent by weight of thecrushed oil shale will pass through a U.S. Sieve Series size 6 screen.

The crushed oil shale may or may not be preheated by direct or indirectheat from any source including indirect heat exchange with pellets orflue gases generated during this retorting process. If the shalefeedstock is preheated, the temperature of the feedstock will not exceed600F. The shale feedstock will usually be fed by way of a metered weightcontroller system, for reasons hereinafter made apparent, and which mayinclude a preheat and/or gas lift system. The preferred system forpreheating the raw shale is to lift the shale in lift pipes with the hotflue gases generated in the combustion phase of the process.

The hot special heat-carrying pellets are especially characterized byhaving a principal size during use of between approximately 0.055 and0.5 inch, and preferably between 0.055 and 0.375 inch, and a surfacearea during use of between 10 and 150 square meters per gram. Thesurface area is the average effective surface area of the pellets asthey enter the pyrolysis zone. The surface area may be determined by theconventional nitrogen absorption method. In one embodiment of theprocess of this invention, the surface area of the pellets on a grambasis is between 10 and 100 square meters. The importance of surfacearea is hereinafter discussed in detail. The heatcarrying pellets are ata temperature ranging between l,000F and 1400F which is about 100F to500F higher than the designed retort temperature within the retort zone.The most favorable practical temperature range depends on "the processvariables and more particularly on the specific advantages andcharacteristics of this process. The quantity of pellet heat carriers iscontrolled to coact with other variables so that the pellet-to-shalefeedstock ratio on a weight basis is between 1 and 3. This ratio is,moreover, such that the sensible heat in the pellets is sufficient toprovide at least per cent of the heat required to heat the shalefeedstock from its retort zone feed temperature to the designed retorttemperature. The feedstock feed temperature is the temperature of theoil shale after preheating, that is the temperature of the shale uponentry into the retort. The average retort temperature ranges betweenabout 850F and l,200F depending on the nature of the shale feedstock,the pellet-to-shale ratio, the type of product distribution desired,heat losses, and the like.

The relative mass and size of the pellets are selected in a mannerhereinafter set forth which facilitates separation of the pellets fromspent shale, controls the amount of combustible deposition deposited onthe pellets, optimizes other facets of the retorting process, and makesallowance for wear or size reduction of the pellets as they are cycledand recycled through the retorting process.

The term pellets refers to subdivided or particulate bodies. A majorityof the bodies have the characteristics and properties herein requiredand which are composed of the same or dissimilar materials having thespecified surface area and strength and of irregular shape, cylindricalshape, approximately oval or spherifrom other solids produced in theprocess as hereinafter set forth. The sphericity factor is the externalor geometric surface area of a sphere having the same volume as thepellet divided by the extemal'surface area of the pellet.

The pellets are made up of materials, such as alumina or silica alumina,which are not consumed in the process and which are subdivided orparticulate matter having significantly high internal surface area butnot excessively high. The pellets are sufficiently wear or breakageresistant and heat resistant to maintain enough of their physicalcharacteristics under the conditions employed in the'process to satisfythe requirements herein set forth, to affect retorting of the oil shale,and to permit controlled burning of a carboncontaining deposition formedon the pellets during the process. More specifically, the pellets do notdisintegrate or decompose, melt or fuse, or undergo excessive surfacearea reduction at the temperatures encountered during such burning andthe thermal stresses inherent in the process. The pellets will, ofcourse, undergo some gradual wear or size reduction.

As will be shown, the size of the pellets is related to the otherprocess variables and to the preburn spent shale separation step of thisprocess. The original or fresh pellets are generally comprised ofparticulate sensible heat carriers in a size range between about 0.1inch and 0.5 inch, and preferably between 0.1 and 0.375 inch, and arefor the most part maintained during use at a plus 14 US. Sieve SeriesScreen size, that is, approximately 0.055 inch or greater. Finer pelletgrain sizes are undesirable in the process of this invention.

Suitable pellet materials are also found in cracking catalyst; however,the retorting process of this invention is not to be considered asrelying on active catalytic sites. Many catalysts have surface areas farin excess of the maximum surface area of 150 square meters per gramprovided in this process. For example, some silica alumina catalyst mayhave a surface area ranging between 180 and 700 square meters per gram.As will hereinafter be discussed and as indicated by the trend shown inTABLE 1, high surface areas tend to cause too much carbon-containingdeposition being deposited in the retort zone.

Active catalytic sites tend to have effects similar to excessively highsurface areas. As a result, in this process, although cracking catalystsmay be used, it is preferred that the pellets bear no added active acidcracking catalyst sites or the like when the pellets are added to theretorting zone. What is preferred are pellets that have the size andsurface area limitations herein set forth. Of course, the retortingphase of this process and the subsequent deposition combustion phasecould be conducted with a catalyst with some loss of flexibility in sucha manner as to kill or limit active catalyst sites and limit or destroyexcessive available pellet surface area; but it is preferred that thepellets not bear such sites and have or rapidly develop the prescribedsurface area range naturally. Thus, the pellets could be comprised ofparticulate or subdivided matter, for example, catalyst particles,composed or manufactured of materials which can be treated to reducetheir surface area and which are of appropriate size, but whichoriginally had a surface area in excess of 150 square meters per gram,and which have been treated to reduce the effective surface area to lessthan 150 square meters per gram. An originally high surface area can bepermanently reduced by methods similar to the way that catalystparticles lose their effective surface area as they age when used incatalytic cracking or hyrogenation units, or by subjecting the particlesto rapid or prolonged aging at temperatures and fluid pressuressufficient to reduce the surface area of the particles. A preferred wayto cause this reduction in surface area is to subject the particles totemperatures above 1,400F and in the presence of steam at pressuresbetween 0.5 and 7 atmospheres until the surface area is reduced to thedesired level. By way of illustration, it has previously been reportedin accelerated aging experiments that by subjecting a silica-aluminacatalyst to one atmosphere of steam for one hour at 1,585F the surfacearea was reduced from about 180 square meters per gram to about 95square meters per gram, and in a similar experiment at 1,432F thesurface area was reduced from about 400 square meters per gram to aboutsquare meters per gram. The high surface area particulate matter thustreated may originally have been comprised of high surface areaparticles with active acid catalytic sites. In such case, the particlescould also be treated to deactivate their active acid catalytic sites bysubjecting them to conditions and chemicals known to poison or kill suchactive acid catalytic sites, for example, by treatment with sodiumbicarbonate, sodium hyroxide, or sodium carbonate.

The retort zone is any sort of retort which causes intimate contact ormixing of the crushed oil shale and pellets. The preferred retort is anysort of horizontal or inclined retorting drum that causes the oil shaleand pellets to undergo a tumbling action. This sort of retort is hereinreferred to as a rotating retort zone. This type of retort zone is quiteflexible over a wide range of conditions and has the advantages ofcausing rapid solidto-solid heat exchange between the pellets and shalefeedstock thereby flashing and pyrolyzing the oil and gas vapors fromthe shale in a way which allows the vapors to separate from the solidswithout passing up through a long bed of solids and which minimizesdilution of the product vapors by extraneous undesirable retortinggases; of allowing for a high shale throughput rate at high yields for agiven retort volume; of providing for greater control over residencetime; of aiding in preventing overcoking and agglomeration of thepellets and shale; of facilitating formation of a more uniformcontrolled amount of combustible carbon-containing deposition on thesurface area of the pellets; and of causing flow of the pellets andshale through the retort zone in a manner which aids in eventualseparation of the pellets from the spent shale. The amount of depositiondeposited on the pellets during the retorting stage of the process is animportant feature and will be discussed later in more detail. Theretorting process is carried out in concurrent or parallel flow fashionwith the hot pellets and the raw shale feedstock being fed into the sameend of the retort. The retort zone may be maintained under any pressurewhich does not hamper efficient operation of the retort, interfere withproduction of valuable retort vapors, or cause excessive deposition ofresidue on the pellets. Generally, pressurization of the pyrolysis orretort zone causes considerable difficulties, especially if a rotatingretort zone is used. The pressure employed is, therefore, generally theautogenous pressure.

In the retort zone, the hotter pellets and cooler crushed shalefeedstock are admixed and intimately contacted almost immediately uponbeing charged into the retort zone. The shale particles are rapidlyheated by sensible heat transfer from the pellets to the shale. Anywater in the shale is distilled and the kerogen or carbonaceous matterin the shale is decomposed, distilled, and cracked into gaseous andcondensable oil fractions, thereby forming a valuable vaporous effluentincluding gas, oil vapors, and superheated steam. Pyrolysis andvaporization of the carbonaceous matter in the oil shale leaves aparticulate spent shale in the form of the spent mineral matrix matterof the oil shale and relatively small amount of unvaporized or cokedorganic carbon-containing material.

As the aforementioned vaporous effluents are formed, a combustiblecarbon-containing deposition or residue will be formed or deposited onthe pellets if the effective surface area of the pellets has not alreadybeen covered with all of the deposition that it can sustain. Thevariables and stages of this process as herein set forth are related ina manner which controls the total amount of combustible deposition thusdeposited during the retort stage of the process and the amountdeposited during a preretort coking or thermal cracking stage of theprocess. The total amount of deposition formed or deposited on thepellets upon one passage through the process is sufficient uponcombustion to provide at least 50 per cent of the heat required to reheat the pellets. The amount of combustible deposition deposited on thepellets during the retorting stage is on an average less than 1.5 percent by weight of the pellets and the preferred range is between 0.8 and1.5 per cent. Basically, these controls are critical in two respects.First, the total amount of deposition on the pellet is important since,as will hereinafter be shown, this deposition is burned in a controlledmanner to generate a major portion of the heat necessary for heating thepellets to carry out the retorting phase of the process. Second, thetotal amount of deposition affects the relative yields of gas andcondensable or final liquefied products. This in turn affects thedistribution of various boiling point fractions in the liquefiedproducts. The total amount of combustible deposition deposited isbasically regulated in this process by a precoking product cracking orstabilization stage as hereinafter described and by the amount ofdeposition deposited on the pellets in the oil shale retorting stage,which deposits are determined by the interrelation of several variables,such as pellet-to-shale ratio, pellet size and surface area, thepercentage of the pellets passed through both the precoking zone and theretort zone, the amount and types of products cracked or stabilizedrelative to the percentage of pellets passed through both the precokingzone and the retort zone, temperatures in the precoking zone and in theretort zone, the outlet temperatures of the precoking cracking stage andthe retort zone, and thetype of retort zone. Additional control overboth the total amount of deposition deposited on the pellets may beobtained by residence time and throughput rate of the precoking zone andin the retort zone, partial or complete combustion of the deposition,controlled deposition combustion'time' or amount of oxidizing gas usedduring burning, the noncatalytic characteristics of the pellets, and thesize of the pores at the surface of the pellets. As can be readily seenby this description of the process, the degree of regulation or controlprovided by a single variable is never independent and the flexibilityof regulation varies with the type of variable.

The pellet surface area is considered one of the most importantvariables. The effect of pellet surface area is illustrated by the testresults set forth in TABLES l, 2, and 3. The effect of pellet surfacearea on the amount of carbon-containing deposition formed on pellets andon distribution of carbon deposition between the pellets and spent shalewithout the precoking thermal stabilization or cracking stage isillustrated in TABLE 1. The effect of pellet surface area on liquidproduct distribution when a modified Fischer retort was used is i1-lustrated in TABLE 2. The effect of pellet-toshale ratio and, therefore,total surface area of the pellets is illustrated in TABLE 3. The totalsurface area is determined by the surface area per gram of pellets andthe total pellet weight which in turn is controlled by thepellet-to-shale ratio and shale throughput rate. The results illustratedin these. tables lead to several conclusions. First, if the surface areaexceeds square meters per gram, too much deposition may be produced onthe pellets during the retorting stage of the process when thepellet-to-shale ratios specified herein are used. This in turn indicatesan undesirable or excessive shift toward gaseous products in the retortzone. In this process, the total amount of deposition formed on thepellets during the retorting stage may also be partially altered bypassing pellets from the thermal precoking stage to the retort zone.

If the surface area of the pellets is less than 10 square meters pergram, either too little total deposition will be formed or the burningof the deposition will not be sufficient to provide a major portion ofthe heat required to heat the pellets to the desired temperature and tocarry out the retorting phase of this process. This would necessitatethe use of supplementary fuels and as indicated previously, this hassignificant disadvantages to the objects of this process.

TABLE 1 EFFECT OF PELLET SURFACE AREA ON CARBON DEPOSITION PELLET AREAWT. PERCENTAGE OF CARBON ON ON PELLETS RESIDUAL SHALE No Pellets 4.30

TABLE 2 EFFECT OF PELLET SURFACE AREA ON LIQUID PRODUCT DISTRIBUTIONPRODUCT NO PELL ET AREA BOILING RANGE PELLETS 47m /g 96mlg 150 400F 12%27% 34% 400 700F 37% 46% 48% 700 900F 32% 22% 14% 900F 19% 5'7: 47:

Pellet Shale Ratio 211 TABLE 3 EFFECT OF PELLET SHALE RATIO ON LIQUIDPRODUCT DISTRIBUTION PRODUCT NO PEILLET SHALE RATIO BOILING RANGEPELLETS 1:! 1.5:] 2:1

150 400F 14% 25% 30% 34% 400 700F 38% 45% 47'?! 48% 700 900F 31% 22% 17%14% 900F 17% 8% 6% 4% by the preretorting or precoking thermal crackingor stabilization stage of this process. In other words, the totaleffective surface area is not only determined by the original surfacearea of the pellets and the amount of pellets, but also by the amount ofcoke or deposition formed on the pellets in the thermal cracking orstabilization stage prior to feeding some or all precoked pellets to theretort zone. As a result, the operator has additional leeway whenselecting the pellet-to-shale ratio and the original surface area of thepellets. All variables considered, it has been concluded that anoriginal pellet surface area between and 150 square meters per gram isacceptable with the range of 10 to 100 being preferred and thatoperating with a pellet-to-shale ratio between 0.5 and 3.5 is feasiblewith a ratio between I and 3 being preferred.

The mixture of pellets and shale moves through the retort zone towardretort exit 17 and the gaseous and vaporous effluents containing thedesired hydrocarbon values separate from the mixture. Since there is noneed to use carrier, fluidizing or retorting gases in the retort zone,the gaseous and vaporous effluents are able to leave the retortessentially undiluted by extraneous fluids except for any water or steamvapor added to prevent or retard carbonization, or to sweep productvapors from the solids, or for other reasons to the retort or effluentcollection chamber. In a rotating retort system, the mixture movement iscontinuous and is aided by the action or design of the retort and bycontinuous withdrawing of pellets and spent shale from the exit end ofthe retort zone. If a rotating retort zone is used, caking or cokingtogether of the heat-carrying pellets or spent shale will be kept low.Moreover, a rotating type of retort zone is especially suited to varyingthe residence time, that is, the length of time that the shale andpellets remain in the retort zone by allowing variations inpellet-to-shale ratio and volume of shale throughput. As previouslyindicated, greater than normal leeway in control over these variables isespecially advantageous to regulation of the amount of depositiondeposited on the pellets during the retorting stage of the process. Theresidence time for the pellets required to effect retorting anddeposition of the pellet deposition is on the order of about 3 to aboutminutes with residence times of less than 12 minutes for the pelletsbeing preferred. The shale residence time depends on its flow ormovement characteristics and since the shale is not uniform in size andshape, the shale residence time varies.

The mixture of pellets and spent shale exits from retort zone 13 at atemperature between 800 and 1,050F by way of retort exit 17 intoseparation zone 19 for separation of the vapor, pellets, and spentshale. The separation zone may be any sort of exiting and separationsystem accomplishing the functions hereinafter mentioned and may becomprised of any number of units of equipment for separating andrecovering one or more of these three classes of retort zone effluentseither simultaneously, or in combination, or individually. In theprocess of this invention, it is critical that at least 75 per cent ofthe total spent shale be separated from the pellets in the separationzone to eventually be collected in separation zone exit line 21. Inaddition, at least 95 per cent of the spent shale smaller than thepellets, that is, smaller than 0.055 inch, are separated. As shown inFIG. 2, the retort zone mixture is first passed through revolving screenor trommel 23 which has openings or apertures sized to pass the pelletsand spent shale of the same or smaller size than the pellets. Thetrommel extends into product recovery chamber 25. In the trommel, thegaseous and vaporous products separate from the mixture of pellets andspent shale and, at the same time, at least a portion of the largerspent shale particles or agglomerates are separated from the pellets andspent shale. Most of the spent shale and the pellets flow through theopenings in trommel 23 and drop to the bottom of recovery chamber 25 toexit via retort exit line 27. Any spent shale or agglomerates too largeto pass through the openings in the trommel pass outward through exit29. The product vapors and gases resulting from retorting the oil shalecollect overhead in recovery chamber 25 and rapidly pass to overheadretort products line 31 at an exit temperature between about 750 and1,050F where the product vapors may or may not be divided into twostreams either before or after the vapors are subjected either in theirvaporous or condensed or partially condensed state to hot dustseparation (not shown) and/or fractionation or partial fractionation(not shown), and/or other stages (not shown) of the overall operation.The hot dust separation may be interior or exterior, or both, ofrecovery chamber 25 and the dust thus collected may be combined andhandled with other spent shale. I-Iot dust or fines separation may beaccomplished by hot gas cyclones, quenching and washing, agglomerationwith sludge or a separately condensed heavy product fraction,centrifuging, filtration, or the like. Partial fractionation may beaccomplished by condensing only a high boiling fraction of the vapors,e.g. 900F+ materials.

Regardless of whether any such additional processing step is taken, atleast a part of the product oil collected in overhead line 31 iseventually passed through precoker feed line 33 to thermal precokingzone 35 which will be hereinafter described. For simplicitys sake, feedline 33 is shown directly connected through a metering valve and crackerfeed pump 37 to overhead line 31 in a way which allows all or a part ofthe product to be fed to thermal cracking unit 35.

As mentioned previously, the gases are not diluted by other gases andare, therefore, readily used in the overall shale operation. Some gasmay be needed for supplementary fuel and some for production in theusual manner of hydrogen if hydrogenation is used in the overall shaleoperation. The optimum amount of gas production is just enough tosatisfy these requirements as this process stresses the liquid oilproducts produced in the overall shale operation.

The spent shale and pellets in recovery chamber 25 are discharged viaexit line 27 at a temperature between about 750 and 1,050F where theseparticulate solids are passed or conducted by gravity or other means ofconveyance to gas elutriation system 39 which is a part of separationzone 19. In the elutriation system, a major portion, and more preferablysubstantially all, of the remaining spent shale is separated from thepellets. It is essential that elutriation be accomplished in a way whichretains the desired amount of combustible deposition on the pellets;consequently, the elutriating gas fed by line 41 is a noncombustionsupporting gas. By conducting the process with pellets in the size rangebetween 0.055 and 0.5 inch, and preferably between 0.055 and 0.375 inch,at least percent of the total spent shale may be separated by action ofthe trommel and subsequent gas elutriation at a velocity of between 18and 25 feet per second if most of the raw shale feedstock was crushed toone-half inch. Based on an average of six sieve analyses of the spentshale produced by retorting of minus three-fourths inch shale feedstockin a rotating retort using ceramic onehalf inch balls, about 16 per centby weight (analyses range 8 to 27 percent) of the spent shale isretained on a US. Sieve Series size 14 screen which is in a size rangesimilar to the pellets. Gas elutriation with irregular or cylindricalshaped pellets only separates about 2.0 to 4.0 per cent of this portionof the spent shale from the pellets. Therefore, on an average between 12and 13 per cent of the spent shale is difficult to separate by screeningand elutriation depending on whether the pellets cover the entire sizerange of this part of the spent shale. As mentioned previously,retention of more than 25 per cent of the spent shale interferes withproper operation of the pellet deposition burning zone even if most ofthe spent shale entering the burning zone is originally in the same sizerange as the pellets. Upon combustion, this spent shale woulddisintegrate further to fine ashand cause erratic operation of thecombustion zone. In addition, some allowance is made for spent shale andash buildup as the pellets are cycled and recycled through the process.

Since the spent shale having a size similar to the pellets is difficultto elutriate while the spent shale smaller than the pellets is readilyseparated by elutriation, and practically complete, it is desirable toalter the characteristics of the spent shale or of the pellets toaccomplish a greater degree of separation while holding heat losses inthe pellets to a reasonable level. One way to accomplish this objectiveis to crush at least 95 per cent by weight of the shale feedstock to aminus 6 screen size. This results in a separation of at least 95 percent by weight of the total spent shale from the pellets and the trommelmay also be eliminated. As mentioned previously, crushing to this sizeis costly and normally not done; however, in view of the fact that inthis invention it is essential that the bulk of the spent shale beseparated from the pellets prior to reheating of the pellets, the costof additional'crushing may be justified. Another way to accomplish theobjective of this separation prior to reheating the pellets has beendiscovered. It has been found that if the pellets are essentiallyspherical, that is, have a sphericity factor of at least 0.9, theefficiency of separation by gas elutriation is greatly increased whenthe raw shale is crushed to a minus three-fourths size. Sphericalpellets have improved flow properties over the spent shale and for agiven screen size particle exhibit greater weight per particle. Gaselutriation with spherical pellets will separate about 97 per cent ormore of the spent shale retained on a US. Sieve Series size 14 screenand will provide almost complete separation of the smaller spent shale.Thus, if spherical pellets are used, gas elutriation will separate atleast 95 per cent of the spent shale in the separation zone. Asmentioned previously, therefore, the preferred shape of the pellets isspherical, that is, the preferred pellet should have a sphericity factorof at least 0.9.

The separated spent shale is carried out of the elutriating chamberoverhead through line 43. The spent shale is collected and may becombined and handled with other spent shale for eventual compaction andwaste disposal or sale for use in manufacturing other products.

The separated pellets with their combustible deposi tion are then passedfrom the separation zone to a pellet deposition burning zone via pelletreturn line to pellet lifting system 47 where the pellets are liftedpreferably to an elevation which allows gravity feed to retort zone 13by way of lift line 49 to pellet deposition burning zone 51, which asShOWfll in FIG. 2 has surge hopper 53 for collecting the lifted pelletsand leveling out fluctuations and from which the pellets fall into pellet deposition burning zone 55. While any conveying and lifting systemholding heat losses to a reasonable value may be used, it is preferredas shown in FIG. 2 that the pellet lifting system be a pneumaticconveying system which will operate in the conventional manner to liftthe pellets to the pellet deposition burning zone. The lift gas entersthe lift system via line 56 at a velocity between 25 and 70 feet persecond and the lift time is,

.therefore, very short. As a result, air may be used as the lift gaswithout causing uncontrolled combustion of the deposition on the pelletsand the detrimental effects attendant to such uncontrolled burning.

As mentioned previously, the pellets bear a combustible deposition whichwas absorbed or deposited during the process. This combustibledeposition is burned in combustion or pellet deposition burning zone 55to provide at least per cent of more of the heat re quired to reheat thepellets to the temperature required to effect retorting of the shale.The combustible deposition is burned in a manner similar to the way thatcracking catalysts particles are regenerated and which is controlled toavoid excessive heating of the pellets which would excessively reducethe effective surface area of the pellets to less than 10 square metersper gram.- A progressive bed burner with a gas flow of about 1 to 2 feetper second is preferred. A combustible supporting gas, for example air,a mixture of air and fuel gas generated in the process, flue gas withthe desired amount of free oxygen, is blown into the pellet depositionbuming zone at a temperature at which the deposition on the pellets isignited by way of combustion gas inlet 57 which lllFIGyZ includes ablower. Steam may also be used to controlbuming provided that the steamdoes not excessively reduce the surface area of the pellets. Thecombustion supporting gas may be preheated in heaters 59 by burning someof the gases produced in the process to reheat the pellets t0 theminimum ignition temperature. The quantity of combustion supporting gas,e.g., about 10 to 15 pounds of air perv pound of deposition, affects thetotal amount of deposition burned and the heat generated by such burningand in turn the temperature of the pellets. The bulk density of thepellets is about 40 to 50 pounds per cubic foot and the specific heat ofthe pellets varies between about 0.2 and 0.3 British Thermal Units perpound per degree Farenheit. The gross heating value of thecarhon-containing deposition is estimated to be about 15,000 to 18,000BTU per lb. The amount of carbon dioxide and carbon monoxide produced.in the flue gases created by burning the pellet deposition indicate theamount of combustion supporting gas required or used and the amount ofcarbon-containing deposition not burned. Generally. it is desirable toattempt to free the pellets of deposition. In any case, at least 50 percent of the deposition is burned. The unburned deposition stays with thepellets and affects to some degree the total amount of combustibledeposition deposited in the next cycle. It should be noted that thistype of controlled burning does not selectively burn the same amount ofdeposition from every pellet. Other factors taken into considerationduring burning of the pellet deposition are the pellet porosity,density, and size, the burner chamber size and pellet bed size,residence burning time, the desired temperature for the pellets, heatlosses and inputs, the pellet and shale feed rates to the retort zoneand the like. The residence burning time will usually be rather long andup to about to minutes. Combustion of the deposition should becontrolled in a manner which does not heat the pellets to above 1,400F.The hot flue gases generated in the pellet deposition burning zone maybe removed by burning zone exit line 61 and used to preheat cool rawshale feedstock or for heat transfer to any other phase or part of theshale operation. For example, this stream could be fed to a carbonmonoxide boiler and the heat available from the boiler could be used forprocessing product vapors or to drive turbines. Of course, additionalfuel material or gases may be used to supplement burning of the pelletdeposition if this is necessary, but it is to be understood that thepellet deposition supplies the major portion of the sensible heatrequired for retorting the shale and that the variables are set toaccomplish this objective along with the other advantages and objectivesof this process.

A continuous stream of hot pellets having a temperature above 1,000F andnot exceeding 1,400F is thereby produced. The hot pellets pass throughthe pellet deposition burning zone exit line 63 either by gravity and/0rmechanical means. As previously indicated, the rate of passage of thepellets from the combustion zone will be metered or controlled inconventional manners to eventually provide the optimum pellet-tooilshale feedstock ratio to the retort zone. The optimum ratio is governedby the pellet properties, the amount of deposition on the pellets asthey enter the retort zone, the organic content of the raw oil shale,and the other process variables as previously described.

As previously mentioned, at least a portion of the hot pellets andretort oil products are passed or fed to precoking zone 35 which mayconsist of one or more cracking units. When the oil products contact thehot pellets, some of the oil is thermally stabilized and cracked toupgrade the oil and the resulting coke-like deposition is deposited asuseful fuel material for heating the pellets. The amount ofstabilization, cracking, and deposition is primarily dependent on thetemperature in the precoking zone; on the surface area of the pellets;the pellet-to-oil ratio; on the nature of the oil products passed to theprecoking zone; and on the space velocity and pellet holding time. Forpractical reasons, once a system has been placed in operation, theresults of the precoking zone are adjusted by the oil product feed rate,the pellet rate, the temperature, or any combination thereof. Since thepellets have just exited from the pellet deposition burning zone, thetemperature may be varied over a wide range. The pellet holding time canbe changed by altering pellet feed rate or by varying the total pelletcharge in the precoking zone. The total pellet charge can be varied byadding or removing cracking units or by decreasing the pellet charge toeach unit. The oil feed rate can, of course, be changed at will. Thepellet feed rate can be varied, but

there is less leeway for change unless the holding time is changed orunless the holding time is changed and pellets are removed from thecycle. It is usually uneconomical to attempt to change the nature of theoil feed, the pellet surface area. and the like. Originally, the natureof the oil feed can be preset by fractionating the retort zone oilproducts; but generally, once a fractionating unit is in operation, itis undesirable to attempt to change the nature of the oil feed bychanging the fractionating unit. The preretort thermal cracking zone.therefore, provides flexibility and adaptability in regulating the totalamount of deposition formed on the pcllets during the process bycontrolling and/or adding to the deposition formed on the pellets duringthe retorting stage of the process. This in turn allows more leeway inoperation of the retort zone.

Accordingly, as illustrated in the drawings, the hot pellets in line 63can either pass by way of precoker feed line 65 to precoker zone 35 orbypass the precoking stage by way of bypass line 67 directly to pelletinlet pipe 15. Ofcourse, it is to be understood that the pelletdeposition burning zone could be comprised of more than one zone whichcould be operated under different conditions or which could exit by wayof separate lines which could in turn be used as separate feed lines toprecoking zone 35 or as separate cracking zone bypass lines.

The hot pellets from the pellet deposition burning zone are availablefor entry into the precoking or thermal cracking zone at any temperatureup to the exit temperature of the pellet deposition burning zone. Thetemperature may, therefore, be selected to thermally stabilize or cracka portion of the oil products which are fed into precoking zone 35 byway of precoker feed line 33 and which are passed in the usual reactormanner over and into contact with a bed of pellets fed to the unit. Theoil products passed to the supplemental deposition zone may if desiredbe preheated by indirect heat exchange located either outside or insidethe precoking zone. Generally, it is best to preheat the oil products ifthe products are substantially cooler than the pellets. For example, theoil products may have been derived from a fractionating unit.

It has been discovered that a preferred oil feed for the precoking zoneis the portion of the retort zone products within the boiling pointbetween lO0F and 700F. Higher boiling point feeds tend to createproblems, and reduce flexibility and reliability of use of thesupplemental deposition as a control over the overall retorting process.An oil feed has a boiling point range between 100F and 700F if at leastper cent of the feed has a boiling point within this range. Narrower oilcuts within this range may be selected.

As mentioned previously, it is highly desirable that the precoking zonebe operated under conditions such that the oil products fed to the zoneundergo significant thermal stabilization and some thermal cracking sothat the deposition formed in the supplemental deposition zone will becomprised essentially of coke-like products. This limits product lossesfor fuel on the pellets to coke-like residues and at the same timeprovides additional stabilization and upgrading of the selectedproducts. Oil feed with above-mentioned preferred boiling point rangeare especially useful for this purpose.

As mentioned previously, the chief factors affecting the amount ofstabilization and cracking of the feed and the amount ofcombustibledeposition deposited on the precoker pellets are the properties of theproducts fed to the unit, the space velocity, the pellet holding time inthe precoking unit, the rate of feed and effective surface area of thepellets, and the average temperature in-the cracking or precoking zone.Under thermal cracking or thermal stabilization conditions, therefore,the space velocity may be varied to control the amount of combustibledeposition formed on the pellets passed to the precoking zone and on thedegree of cracking and stabilization of the oil products treated. Thespace velocity is herein defined as the ratio of the pounds of oilproducts passed to the precoking zone per hour to the pounds of pelletsin the precoking zone. As shown in TABLE 4, when a composite naphtha oilfraction from a ball type of oil shale retort with a selected boilingrange between 100F and 700F was thermally treated at 900F, smallincremental changes in space velocities from 2.5 to 0.25 and lower causea substantial change in the amount of carbon deposited on the pelletspassed to the precoking zone. With less desirable higher boiling oilfeeds, even lower space velocities must be used.

TABLE 4 position burning zone when the combustible deposition on thepellets is burned.

EXAMPLE A retort train operating at a poundsof-pellets topounds-of-shale ratio of 2:1 and charging pellets with an effectivesurface area of 46 square meters per gram processes 458.3 tons per hourof raw oil shale. Hot pellets exit a pellet deposition burning zone at1,300F and at a rate of 1,904,533 pounds per hour. The hot pellets aredivided into two streams with 1,833,333 pounds per hour of pellets at anaverage temperature of about 1,300F entering a retort zone and with71,200 pounds per hour at about 1,300F being passed to one or moreprecoking units containing a total pellet charge of 35,600 pounds. Rawoil shale preheated to 450F is fed to the retort at the rate of 916,667pounds per hour with the hot pellets. The hot pellets provide the heatnecessary to retort the preheated oil shale. A combustible deposition of1.24 weight per cent is deposited on the pellets in the retort. Thepellets and processed shale exit the retort zone into a separation zonewhere about THERMAL TREATMENT OF COMPOSITE NAPHTHA AT 900F Endpoint ofthe Treated Product Maleic Carbon Deposit Space Velocity and CumulativeWt. Percent Anhydride On Pellets Treated No. Feed/hr/No. Pellets'" 4Boiling Below Endpoint Numher" Wt. Percent" 200F 300F 400F 500F PelletHolding time was 30 minutes. Values are approximate values. Values for0.25 Space Velocity are extrapolated values.

As illustrated by TABLE 4, the cumalative weight per cent at differentendpoint boiling points of the cracked, stabilized product from theillustrative precoking zone shows that as .the space velocity isdecreased the oil fraction undergoes a greater degree of cracking andstabilization. The maleic anhydride number is a standard test used toindicate conjugate diolefins which in turn is an indicator of the degreeof stabilization of the thermally treated product. Since the ma-' Ieicanhydride number is the milligrams reacted per gram of feed, a decreasein maleic anhydride number indicates a greater degree of stabilization.As the space velocity decreases the product treated in the precokingzone is made more stable. t

As a result. the oil product feed is thermally stabilized or crackedproducing coke or residue on the pellets and an upgraded product whichpasses to cracking zone product line 69. The precoked pellets, which mayhave been swept or stripped of products, are passed by gravity or othermeans through precoker exit line 71 to either pellet inlet line 15 andinto pyrolysis zone 13m to bypass the retort zone via bypass line 73 topellet return line 45 where they are carried to pellet depositionburning zone SI. The amount of combustible deposition deposited on thepellets duringthe precoking or thermal stabilization stage should beless than five per cent by weight of the pellets passed through thesupplemental deposition zone. This avoids overheating of the treatedpellets and localized hot spots in the pellet de- 98 per cent of theprocessed shale is separated from the pellets. The pellets exit theseparation zone at a temperature of about 900F. For illustrationpurposes, a rough naphtha oil having a simulated gas chromatographictrue boiling point distillation showing 35' cumulative weight per centat 350F, 68 cumulative weight per cent at 500F, and 94 cumulative weightper cent at 600F is available for use as an oil feed to the precokingunit. The hot pellets passed to the precoking zone are cooled to 900F,and the rough :naphtha feed passed to the precoking zone is preheated to900F by indirect heat exchange or any other means so that the precokingzone temperature is about 900F. The pellet holding time is about 0.5hour. In one instance, 32,667 pounds per hour of rough naphtha is fed tothe precoking zone. The space velocity is 0.92, and 2.72 per centcombustible fuel than would have been generated if all- .of the pelletshad been passed through the retort zone without the precoking stage. Inanother instance, only 8,900 pounds per hour of rough naphtha is fed tothe precoking zone. The space velocity is 0.25, and 0.78 per cent'byweight of combustible deposition is deposited on the pellets passedthrough the precoking zone. The rough naphtha feed is partiallythermally stabilized and cracked with an estimated reduction maleicanhydride number of from about 21.7 to about 4.5. If the precokedpellets are combined with the pellets going to the retort zone, theprecoking stage will generate about 550 pounds per hour of additionalcombustible deposition for use as fuel for heating the pellets. If theprecoked pellets bypass the retort zone, the total amount of fuel willbe about 330 pounds per hour less than would have been formed if all ofthe pellets had been passed through the retort zone without theprecoking stage.

Although the retorting process is carried out in a manner to hold lossof pellets to a minimum, some pellets will be lost in the process and arelatively small quantity of pellets may be added continuously tomaintain the pellet quantity.

The foregoing description of the conditions and variables under whichthe cracking or precoking stage and the retorting stage can be conductedillustrates how the precoking or pretort thermal cracking coacts withthe retorting stage to accomplish the advantages and objec* tives hereinset forth.

Reasonable variations and modifications are practical within the scopeof this disclosure without departing from the spirit and scope of theclaims of this invention. For example, while the disclosure of thisprocess and the variables have been limited to oil shale, the processconcepts lend themselves readily to retorting any solid organiccarbonaceous material containing hydrocarbon values which can berecovered by thermal vaporization of the solid carbonaceous material,such as, for example, coal, peat, and tar sands. By way of furtherexample, while only a single train of units and stages have beendescribed, it is to be understood that any stage or zone could becomprised of more than one stage or zone, each of which could beoperated under different conditions to provide the overall combinedeffect set forth.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a method for retorting of crushed oil shale containingcarbonaceous organic matter and mineral matter wherein pellets have beenheated in a burning zone to a burning zone exit temperature of between1,000F and 1,400F mainly by a combustion of a combustiblecarbon-containing deposition on said pellets, and wherein oil shale isretorted in a retort zone to gas and oil products, and solid particulatespent shale by contacting said oil shale with a sufficient amount ofsaid heated pellets passed to said retort zone to provide at least 50per cent of the heat required to vaporize a major portion of thecarbonaceous matter from said oil shale and to heat said crushed oilshale from its retort zone inlet temperature to a retort zone outlettemperature of between 800F and l,l50F, and wherein said gas and oilproducts are separated and recovered, the improvement comprising passingat least a portion of said heated pellets from said heating zone to athermal precoking zone and at the same time passing at least a portionof said oil products to said thermal precoking zone into contact withsaid pellets in said thermal precoking zone to effect some thermalstabilization and cracking of said oil products and to at leastpartially deposit a combustible coke-like deposition on the pelletspassed to said thermal precoking zone, and thereafter eventually passingthe pellets with said combustible coke-like deposition deposited thereonfrom said thermal precoking zone to said burning zone.

2. The method according to claim 1 wherein said pellets are comprised ofparticulate solid heat carriers in a size range between approximatelyabout 0.055 inch and 0.5 inch and have a surface area of between 10 and150 square meters per gram of pellets, and the amount of said heatedpellets passed to said retort zone is such that the ratio of said heatedpellets to said crushed oil shale in said retort zone on a weight basisis between one and three, and wherein at least percent by weight of thetotal of said spent shale and at least percent by weight of the portionof said spent shale that is smaller in size than said heated pellets isseparated in a separation zone from said heated pellets after retortingof said oil shale but prior to said heating of said pellets bycombustion of said deposition on said pellets.

3. The method according to claim 1 wherein the particulate solid heatcarriers are in a size range between approximately about 0.055 inch and0.375 inch.

4. The method according to claim 1 wherein at least a portion of thepellets from said thermal precoking zone is passed through said retortzone prior to eventually being passed to said burning zone.

5. The method according to claim 4 wherein said pellets are comprised ofparticulate solid heat carriers in a size range between approximatelyabout 0.055 inch and 0.5 inch and have a surface area of between 10 and150 square meters per gram of pellets, and the amount of said heatedpellets passed to said retort zone is such that the ratio of said heatedpellets to said crushed oil shale in said retort zone on a weight basisis between one and three, and wherein at least 75 percent by weight ofthe total of said spent shale and at least 95 percent by weight of theportion of said spent shale that is smaller in size than said heatedpellets is separated in a separation zone from said heated pellets afterretorting of said oil shale but prior to said heating of said pellets bycombustion of said deposition on said pellets.

6. The method according to claim 1 wherein the particulate solid heatcarriers are 'in a size range between approximately about 0.055 inch and0.375 inch.

7. The method according to claim 1 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.

8. The method according to claim 1 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between F and700F.

9. A method for retorting of crushed oil shale containing carbonaceousorganic matter and mineral matter comprising a. feeding crushed oilshale and pellets to a retort zone, said pellets being comprised chieflyof particulate heat carriers being in a size range between 0.5 inch andapproximately 0.055 inch and having a surface area of between 10 andsquare meters per gram of pellets, said pellets being at a retort zoneinlet temperature between l,000F and 1,400F and in a quantity such thatthe ratio of said heat-carrying pellets to said crushed oil shaleentering said retort zone on a weight basis is between 1 and 3, saidratio also beingsuch that the sensible heat in said pellets issufficient to provide at least 50 per cent of the heat required to heatsaid crushed oil shale from its retort zone feed tempera ture to aretort zone outlet temperature of between 800F and 1,150F;

b. retorting in said retort zone gas and oil products from said crushedoil shale, thereby forming particulate spent shale;

c. causing said pellets and said spent shale to pass from said retortzone to a particle separation zone and separating from said pellets atleast 75 per cent by weight of the total spent shale and at least 95 percent by weight of the portion of said spent shale that is smaller insize than said pellets, prior to heating said pellets in a pelletdeposition burning zone;

d. recovering said gas and oil products generated by retorting saidcrushed oil shale;

e. passing said pellets from said separation zone to a pellet depositionburning zone; f. heating said pellets in said pellet deposition burningzone to an outlet temperature of between 1,000F and 1,400F by burning acombustible carboncontaining deposition on said pellets with acombustion supporting gas;

g. passing at least a portion of said pellets from said pelletdeposition burning zone to a thermal precoking zone and at the same timepassing at least a portion of said oil products to said thermalprecoking zone into contact with the pellets in said thermal precokingzone to effect partial thermal stabilization and cracking of said oilproducts and to at least partially deposit a combustible coke-likedeposition on the pellets in said thermal precoking zone; and

h. thereafter eventually passing the pellets from said thermal precokingzone with said deposited cokelike deposition thereon to said pelletdeposition burning zone.

10. The method according to claim 9 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.

11. The method according to claim 9 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between 100F and700F.

12. The method according to claim 9 wherein a combustiblecarbon-containing deposition is deposited on said pellets in said retortzone, the average amount of said combustible deposition formed on saidpellets upon passage through said retort zone is on said average lessthan 1.5 percent by weight of said pellets passed through said retortzone.

13. The method according to claim 9 wherein the particulate heatcarriers are in a size range between 0.375 inch and approximately 0.055inch.

14. The method according to claim 9 wherein at least a portion of thepellets from said thermal precoking zone is passed through said retortzone prior to eventually being passed to said pellet deposition burningzone.

15. The method according to claim 14 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 percent by weight of thepellets passed through said thermal precoking zone.

16. The method according to claim 14 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between l00F and700F.

17. The method according to claim 9 wherein the particulate heatcarriers are in a size range between 0.375 inch and approximately 0.055inch.

18. The method according to claim 9 wherein the separation of step (c)is comprised of first passing said pellets and said spent shale throughapertures in a trommel to screen out at least a portion of the spentshale and any agglomerates larger than said pellets, and thereaftersubjecting the remaining pellets and spent shale to gas elutriation witha noncombustible supporting gas to effect further separation of thespent shale from the pellets.

19. The method according to claim 18 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 percent by weight of thepellets passed through said thermal precoking zone.

20. The method according to claim 18 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between 100F and700F.

21. The method according to claim 18 wherein a combustiblecarbon-containing deposition is deposited on said pellets in said retortzone, the average amount of said combustible deposition formed on saidpellets upon passage through said retort zone is on said average lessthan 1.5 percent by weight of said pellets passed through said retortzone.

22. The method according to claim 18 wherein at 24. The method accordingto claim 22 wherein the oil products passed to said thermal precokingzone have a boiling point range between 100F and 700F.

25. The method according to claim 9 wherein the pellets have asphericity factor of at least 0.9 and at least percent by weight of thetotal spent shale is separated from said pellets in step (c).

26. The method according to claim 25 wherein the particulate heatcarriers are in a size range between 0.375 inch and approximately 0.055inch.

27. The method according to claim 25 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.

28. The method according to claim 25 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between 100F and700F.

29. The method according to claim 25 wherein a combustiblecarbon-containing deposition is deposited on said pellets in said retortzone, the average amount of said combustible deposition formed on saidpellets upon passage through said retort zone is on said average lessthan 1.5 per cent by weight of said pellets passed through said retortzone.

30. The method according to claim 25 wherein at least a portion of thepellets from said thermal precoking zone is passed through said retortzone prior to eventually being passed to said pellet deposition burningzone.

31. The method according to claim 30 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than per cent by weight of thepellets passed through said thermal precoking zone.

32. The method according to claim 30 wherein the oil products passed tosaid thennal precoking zone have a boiling point range between 100F and700F.

33. The method according to claim 9 wherein at least 95 percent byweight of the crushed oil shale of step (a) has been crushed to a sizeto pass through a U.S. Sieve Series size 6 screen and at least 95percent by weight of the total spent shale is separated from saidpellets in step (c).

34. The method according to claim 33 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.

35. The method according to claim 33 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between 100F and700F.

36. The method according to claim 33 wherein a combustiblecarbon-containing deposition is deposited on said pellets in said retortzone, the average amount of said combustible deposition formed on saidpellets upon passage through said retort zone is on said average lessthan 1.5 per cent by weight of said pellets passed through said retortzone.

37. The method according to claim 33 wherein at least a portion of thepellets from said thermal precoking zone is passed through said retortzone prior to eventually being passed to said pellet deposition burningzone.

38. The method according to claim 37 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.

39. The method according to claim 37 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between 100F and700F.

40. The method according to claim 9 wherein said pellets have a surfacearea of between 10 and 100 square meters per gram of pellets.

41. The method according to claim 40 wherein the particulate heatcarriers are in a size range between 0.375 inch and 0.055 inch.

42. The method according to claim 40 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.

43. The method according to claim 40 wherein the oil products passed tosaid thermal precoking zone have a boiling point range between l00F and700F.

44. The method according to claim 40 wherein a combustiblecarbon-containing deposition is deposited on said pellets in said retortzone, the average amount of said combustible deposition formed on saidpellets upon passage through said retort zone is on said average lessthan 1.5 per cent by weight of said pellets passed through said retortzone.

45. The method according to claim 40 wherein at least a portion of thepellets from said thermal precoking zone is passed through said retortzone prior to eventually being passed to said pellet deposition burningzone.

46. The method according to claim 45 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.

47. The method according to claim 45 wherein the oil products passed tosaid thennal precoking zone have a boiling point range between 100F and700F.

48. The method according to claim 40 wherein the separation of step (c)is comprised of first passing said pellets and said spent shale throughapertures in a trommel to screen out at least a portion of the spentshale and any agglomerates larger than said pellets, and thereaftersubjecting the remaining pellets and spent shale to gas elutriation witha non-combustion gas to effect further separation of the spent shalefrom the pellets.

49. The method according to claim 40 wherein the pellets have asphericity factor of at least 0.9 and at least per cent by weight of thetotal spent shale is separated from said pellets in step (c).

50. The method according to claim 40 wherein at least 95 per cent byweight of the crushed oil shale of step (a) has been crushed to a sizeto pass through a U.S. Sieve Series size 6 screen and at least 95 percent by weight of the total spent shale is separated from said pelletsin step (c).

1. IN A METHOD FOR RETORTING OF CRUSHED OIL SHALE CONTAININGCARBONACEOUS ORGANIC MATTER AND MINERAL MATTER C WHEREIN PELLETS HAVEBEEN HEATED IN A BURNING ZONE TO A BURNING ZONE EXIT TEMPERATURE OFBETWEEN, 1,000*F AND 1,400*F MAINLY BY A COMBUSTION OF A COMBUSTIBLECARBON-CONTAINING DEPOSITION ON SAID PELLETS, AND WHEREIN OIL SHALE ISRETORTED IN A RETORT ZONE TO GAS AND OIL PRODUCTS AND SOLID PARTICULATESPENT SHALE BY CONTACTING SAID OIL SHALE WITH A SUFFICIENT AMOUNT OFSAID HEATED PELLETS PASSED TO SAID RETORT ZONE TO PROVIDE AT LEAST 50PER CENT OF THE HEAT REQUIRED TO VAPORIZE A MAJOR PORTION OF THECARBONACEOUS MATTER FROM SAID OIL SHALE AND TO HEAT SAID CRUSHED OILSHALE FROM ITS RETORT ZONE INLET TEMPERATURE TO A RETORT ZONE OUTLETTEMPERATURE OF BETWEEN 800*F AND 1,150*F, AND WHEREIN SAID GAS AND OILPRODUCTS ARE SEPARATED AND RECOVERED, THE IMPROVEMENT COMPRISING PASSINGAT LEAST A PORTION OF SAID HEATED PELLETS FROM SAID HEATING ZONE TO ATHERMAL PRECOKING ZONE AND AT THE SAME TIME PASSING AT LEAST A PORTIONOF SAID OIL PRODUCTS TO SAID THERMAL PRECOKING ZONE INTO CONTACT WITH 2.The method according to claim 1 wherein said pellets are comprised ofparticulate solid heat carriers in a size range between approximatelyabout 0.055 inch and 0.5 inch and have a surface area of between 10 and150 square meters per gram of pellets, and the amount of said heatedpellets passed to said retort zone is such that the ratio of said heatedpellets to said crushed oil shale in said retort zone on a weight basisis between one and three, and wherein at least 75 percent by weight ofthe total of said spent shale and at least 95 percent by weight of theportion of said spent shale that is smaller in size than said heatedpellets is separated in a separation zone from said heated pellets afterretorting of said oil shale but prior to said heating of said pellets bycombustion of said deposition on said pellets.
 3. The method accordingto claim 1 wherein the particulate solid heat carriers are in a sizerange between approximately about 0.055 inch and 0.375 inch.
 4. Themethod according to claim 1 wherein at least a portion of the pelletsfrom said thermal precoking zone is passed through said retort zoneprior to eventually being passed to said burning zone.
 5. The methodaccording to claim 4 wherein said pellets are comprised of particulatesolid heat carriers in a size range between approximately about 0.055inch and 0.5 inch and have a surface area of between 10 and 150 squaremeters per gram of pellets, and the amount of said heated pellets passedto said retort zone is such that the ratio of said heated pellets tosaid crushed oil shale in said retort zone on a weight basis is betweenone and three, and wherein at least 75 percent by weight of the total ofsaid spent shale and at least 95 percent by weight of the portion ofsaid spent shale that is smaller in size than said heated pellets isseparated in a separation zone from said heated pellets after retortingof said oil shale but prior to said heating of said pellets bycombustion of said deposition on said pellets.
 6. The method accordingto claim 1 wherein the particulate solid heat carriers are in a sizerange between approximately about 0.055 inch and 0.375 inch.
 7. Themethod according to claim 1 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.
 8. The methodaccording to claim 1 wherein the oil products passed to said thermalprecoking zone have a boiling point range between 100*F and 700*F.
 9. Amethod for retorting of crushed oil shale containing carbonaceousorganic matter and mineral matter comprising a. feeding crushed oilshale and pellets to a retort zone, said pellets being comprised chieflyof particulate heat carriers being in a size range between 0.5 inch andapproximately 0.055 inch and having a surface area of between 10 and 150square meters per gram of pellets, said pellets being at a retort zoneinlet temperature between 1,000*F and 1,400*F and in a quantity suchthat the ratio of said heat-carrying pellets to said crushed oil shaleentering said retort zone on a weight basis is between 1 and 3, saidratio also being such that the sensible heat in said pellets issufficient to provide at least 50 per cent of the heat required to heatsaid crushed oil shale from its retort zone feed temperature to a retortzone outlet temperature of between 800*F and 1,150*F; b. retorting insaid retort zone gas and oil products from said crushed oil shale,thereby forming particulate spent shale; c. causing said pellets andsaid spent shale to pass from said retort zone to a particle separationzone and separating from said pellets at least 75 per cent by weight ofthe total spent shale and at least 95 per cent by weight of the portionof said spent shale that is smaller in size than said pellets, prior toheating said pellets in a pellet deposition burning zone; d. recoveringsaid gas and oil products generated by retorting said crushed oil shale;e. passing said pellets from said separation zone to a pellet depositionburning zone; f. heating said pellets in said pellet deposition burningzone to an outlet temperature of between 1,000*F and 1,400*F by burninga combustible carbon-containing deposition on said pellets with acombustion supporting gas; g. passing at least a portion of said pelletsfrom said pellet deposition burning zone to a thermal precoking zone andat the same time passing at least a portion of said oil products to saidthermal precoking zone into contact with the pellets in said thermalprecoking zone to effect partial thermal stabilization and cracking ofsaid oil products and to at least partially deposit a combustiblecoke-like deposition on the pellets in said thermal precoking zone; andh. thereafter eventually passing the pellets from said thermal precokingzone with said deposited coke-like deposition thereon to said pelletdeposition burning zone.
 10. The method according to claim 9 wherein theaverage amount of said combustible coke-like deposition formed on thepellets in said thermal precoking zone is on said average less than 5per cent by weight of the pellets passed through said thermal precokingzone.
 11. The method according to claim 9 wherein the oil productspassed to said thermal precoking zone have a boiling point range between100*F and 700*F.
 12. The method according to claim 9 wherein acombustible carbon-containing deposition is deposited on said pellets insaid retort zone, the average amount of said combustible depositionformed on said pellets upon passage through said retort zone is on saidaverage less than 1.5 percent by weight of said pellets passed throughsaid retort zone.
 13. The method according to claim 9 wherein theparticulate heat carriers are in a size range between 0.375 inch andapproximately 0.055 inch.
 14. The method according to claim 9 wherein atleast a portion Of the pellets from said thermal precoking zone ispassed through said retort zone prior to eventually being passed to saidpellet deposition burning zone.
 15. The method according to claim 14wherein the average amount of said combustible coke-like depositionformed on the pellets in said thermal precoking zone is on said averageless than 5 percent by weight of the pellets passed through said thermalprecoking zone.
 16. The method according to claim 14 wherein the oilproducts passed to said thermal precoking zone have a boiling pointrange between 100*F and 700*F.
 17. The method according to claim 9wherein the particulate heat carriers are in a size range between 0.375inch and approximately 0.055 inch.
 18. The method according to claim 9wherein the separation of step (c) is comprised of first passing saidpellets and said spent shale through apertures in a trommel to screenout at least a portion of the spent shale and any agglomerates largerthan said pellets, and thereafter subjecting the remaining pellets andspent shale to gas elutriation with a noncombustible supporting gas toeffect further separation of the spent shale from the pellets.
 19. Themethod according to claim 18 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 percent by weight of thepellets passed through said thermal precoking zone.
 20. The methodaccording to claim 18 wherein the oil products passed to said thermalprecoking zone have a boiling point range between 100*F and 700*F. 21.The method according to claim 18 wherein a combustible carbon-containingdeposition is deposited on said pellets in said retort zone, the averageamount of said combustible deposition formed on said pellets uponpassage through said retort zone is on said average less than 1.5percent by weight of said pellets passed through said retort zone. 22.The method according to claim 18 wherein at least a portion of thepellets from said thermal precoking zone is passed through said retortzone prior to eventually being passed to said pellet deposition burningzone.
 23. The method according to claim 22 wherein the average amount ofsaid combustible coke-like deposition formed on the pellets in saidthermal precoking zone is on said average less than 5 percent by weightof the pellets passed through said thermal precoking zone.
 24. Themethod according to claim 22 wherein the oil products passed to saidthermal precoking zone have a boiling point range between 100*F and700*F.
 25. The method according to claim 9 wherein the pellets have asphericity factor of at least 0.9 and at least 95 percent by weight ofthe total spent shale is separated from said pellets in step (c). 26.The method according to claim 25 wherein the particulate heat carriersare in a size range between 0.375 inch and approximately 0.055 inch. 27.The method according to claim 25 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.
 28. The methodaccording to claim 25 wherein the oil products passed to said thermalprecoking zone have a boiling point range between 100*F and 700*F. 29.The method according to claim 25 wherein a combustible carbon-containingdeposition is deposited on said pellets in said retort zone, the averageamount of said combustible deposition formed on said pellets uponpassage through said retort zone is on said average less than 1.5 percent by weight of said pellets passed through said retort zone.
 30. Themethod according to claim 25 wherein at least a portion of the pelletsfrom said thermal precoking zone is passed through said retort zoneprior to eventually beiNg passed to said pellet deposition burning zone.31. The method according to claim 30 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.
 32. The methodaccording to claim 30 wherein the oil products passed to said thermalprecoking zone have a boiling point range between 100*F and 700*F. 33.The method according to claim 9 wherein at least 95 percent by weight ofthe crushed oil shale of step (a) has been crushed to a size to passthrough a U.S. Sieve Series size 6 screen and at least 95 percent byweight of the total spent shale is separated from said pellets in step(c).
 34. The method according to claim 33 wherein the average amount ofsaid combustible coke-like deposition formed on the pellets in saidthermal precoking zone is on said average less than 5 per cent by weightof the pellets passed through said thermal precoking zone.
 35. Themethod according to claim 33 wherein the oil products passed to saidthermal precoking zone have a boiling point range between 100*F and700*F.
 36. The method according to claim 33 wherein a combustiblecarbon-containing deposition is deposited on said pellets in said retortzone, the average amount of said combustible deposition formed on saidpellets upon passage through said retort zone is on said average lessthan 1.5 per cent by weight of said pellets passed through said retortzone.
 37. The method according to claim 33 wherein at least a portion ofthe pellets from said thermal precoking zone is passed through saidretort zone prior to eventually being passed to said pellet depositionburning zone.
 38. The method according to claim 37 wherein the averageamount of said combustible coke-like deposition formed on the pellets insaid thermal precoking zone is on said average less than 5 per cent byweight of the pellets passed through said thermal precoking zone. 39.The method according to claim 37 wherein the oil products passed to saidthermal precoking zone have a boiling point range between 100*F and700*F.
 40. The method according to claim 9 wherein said pellets have asurface area of between 10 and 100 square meters per gram of pellets.41. The method according to claim 40 wherein the particulate heatcarriers are in a size range between 0.375 inch and 0.055 inch.
 42. Themethod according to claim 40 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.
 43. The methodaccording to claim 40 wherein the oil products passed to said thermalprecoking zone have a boiling point range between 100*F and 700*F. 44.The method according to claim 40 wherein a combustible carbon-containingdeposition is deposited on said pellets in said retort zone, the averageamount of said combustible deposition formed on said pellets uponpassage through said retort zone is on said average less than 1.5 percent by weight of said pellets passed through said retort zone.
 45. Themethod according to claim 40 wherein at least a portion of the pelletsfrom said thermal precoking zone is passed through said retort zoneprior to eventually being passed to said pellet deposition burning zone.46. The method according to claim 45 wherein the average amount of saidcombustible coke-like deposition formed on the pellets in said thermalprecoking zone is on said average less than 5 per cent by weight of thepellets passed through said thermal precoking zone.
 47. The methodaccording to claim 45 wherein the oil products passed to said thermalprecoking zone have a boiling point range between 100*F and 700*F. 48.The method according to claim 40 wherein the separation of step (c) iscomprised of first passing said pellets and said spent shale throughapertures in a trommel to screen out at least a portion of the spentshale and any agglomerates larger than said pellets, and thereaftersubjecting the remaining pellets and spent shale to gas elutriation witha non-combustion gas to effect further separation of the spent shalefrom the pellets.
 49. The method according to claim 40 wherein thepellets have a sphericity factor of at least 0.9 and at least 95 percent by weight of the total spent shale is separated from said pelletsin step (c).
 50. The method according to claim 40 wherein at least 95per cent by weight of the crushed oil shale of step (a) has been crushedto a size to pass through a U.S. Sieve Series size 6 screen and at least95 per cent by weight of the total spent shale is separated from saidpellets in step (c).